High flyer listens for neutrinos' pings

At 13.40 on 15 December, a giant balloon carrying an equally gigantic payload lifted off from McMurdo station in Antarctica. Steve Barwick was glad to see it go. Maybe 'twas the season, but the device he had spent two years building had begun to remind him of a Christmas tree. However, the adornments on this 6-metre-tall hulk are radio antennas rather than baubles. And far from being decorative, this device is now turning the ice continent into the world's biggest particle detector.

Barwick, of the University of California, Irvine, and colleagues developed ANITA - the Antarctic Impulsive Transient Antenna experiment - to detect ultra-high-energy neutrinos from sources outside our galaxy. If ANITA succeeds, it will become the first detector to spot such neutrinos. It could also help shed light on the origin of mysterious ultra-high-energy (UHE) cosmic rays, charged particles arriving on Earth at nearly the speed of light.

According to standard physics, cosmic rays created outside our galaxy with energies greater than about 1020 electronvolts (eV) should not reach Earth at those energies: as they travel over such vast regions of space they should lose energy because of collisions with photons of the cosmic microwave background (CMB), the radiation left over from the big bang.

This leads to a maximum possible energy for such cosmic rays reaching Earth, called the Greisen-Zatsepin-Kuzmin (GZK) cut-off. However, in 2004, Japan's Akeno Giant Air Shower Array announced that since 1990 it had detected 11 cosmic rays with energies greater than this cut-off. In contrast, the HiRes cosmic ray detector in Utah did find signs of a cut-off (New Scientist, 14 October, p 14). "We've got a real mystery on our hands," says physicist Peter Gorham, of the University of Hawaii, Honolulu, who heads the ANITA collaboration.

While cosmic-ray detectors such as the Pierre Auger observatory in Argentina are gearing up to look for UHE cosmic rays themselves, physicists have another ally in their quest to solve the puzzle. When cosmic rays hit the microwave photons in space, theory predicts that they should produce neutrinos with energies of about 1018 eV, as well as other particles. Because neutrinos rarely interact with matter, they should travel to Earth unimpeded.

It's these so-called GZK neutrinos that Gorham and his colleagues are trying to detect with ANITA. "We need to find these neutrinos or we have real problems with our understanding of physics," says Gorham, speaking from McMurdo Station in Antarctica. "These neutrinos are no longer optional."

ANITA will exploit a phenomenon known as the Askaryan effect, whereby high-energy neutrinos streaming through ice, salt or sand produce a cone of radio waves when they collide with a nucleus in the material. These materials are transparent to radio waves, so the radio pulses should be detectable.

However, because neutrinos interact so rarely with matter, these collisions are few and far between. To have any chance of spotting them, you need to observe a vast mass of material. "We couldn't find a huge quantity of pure sand or salt, but Antarctica has a huge mass of pure ice," says Barwick. "That decided it."

Before shipping their detector to Antarctica, the team had to prove that the theory works. So, six months ago, at the height of summer, Gorham, Barwick and colleagues found themselves jack-hammering away at a 10-tonne block of ice at the Stanford Linear Accelerator in Palo Alto, California. "It was the hottest day of the year and we were creating a mini-Antarctica from ice used by sculptors for weddings." says Barwick. "I had to laugh."

The team fired artificially generated neutrinos into their ice sculpture and successfully measured radio waves using the ANITA antennas (www.arxiv.org/hep-ex/0611008), which are designed to pick out radio waves from GZK neutrinos and nothing else. "It was vitally important to prove that the antennas work in this way," says Gorham.

ANITA was shipped to Antarctica in September, after which the team had to just wait patiently for the right conditions for launch. Barwick, speaking from McMurdo earlier, said the atmosphere had been "electric".

The balloon will take ANITA up to 38 kilometres over the Antarctic ice cap, where it will circle the South Pole, allowing its antennas to scan a million cubic kilometres of ice at a time (10 to 15 per cent of the whole continent), making this the largest neutrino detector by far. The next largest is IceCube, which will monitor a cubic kilometre of Antarctic ice using buried detectors to look for flashes of light generated by neutrinos hitting the ice.

Because radio waves can travel through ice for long distances without weakening, ANITA will be able to detect neutrino collisions occurring a mile down in the ice, even from 400 kilometres away.

The experiment is scheduled to fly for 40 days, but the time it spends up in the air is down to nature. The team will be monitoring the weather and the balloon carefully. If the balloon begins to veer towards the ocean or mountainous terrain, from where it can't be recovered if it descends, the mission will have to be terminated, says Barwick. The team will detonate an explosive bolt in the wires holding ANITA, so that it can parachute down in an accessible region.

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